This invention relates to and has for its objective the electrochemical treatment of lithographic-grade aluminum webs. More particularly, this invention relates to a method for the liquid contact of an aluminum web by the employment of a sulfuric acid electrolyte. More specifically, this invention pertains to a method for the treatment of an aluminum web in a sulfuric acid electrolyte by the employment of an anode comprised of a titanium substrate to which has been uniformly applied an iridium oxide coating.
In the past, many methods have been devised for the anodization of aluminum. One of the most commonly employed methods for the anodization of aluminum involves the use of sulfuric acid as the anodization electrolyte. See, for example, U.S. Pat. No. 3,891,516. This method of electrochemically treating aluminum is especially preferred in the anodization of aluminum webs in the production of photosensitized lithographic printing plates.
The conventional way for anodizing an aluminum web is by treating it with DC current in an acid electrolyte, e.g., 5-20% sulfuric acid. The anodic terminal for the DC current is connected to a metallic roller which supports the aluminum web before it enters the acid electrolyte. The cathodic terminal is, therefore, connected to a metallic cathode which is dipped in the acid electrolyte and a distance is maintained of about 5 inches away from the aluminum web. When the DC current passes through the electrolysis tank the aluminum will be anodized and an oxide layer forms on its surface, so called anodic oxide. In such a way, the DC current should pass through the contact interface of the metallic roller and the aluminum web. As the anoidizing proceeds, oxides will form on the surface of the metallic contact roller, so that the resistance of the interface increases and electric sparks occur. The sparks will cause specks on the surface of the aluminum web. These specks will stay and cause a problem for the aluminum surface when employed as the base of a lithographic printing plate.
To prevent the electric spark at the contact surface, a liquid contact treatment before or after the anodizing tank, has been proposed. Several patents have been granted to W. E. Cooke U.S. Pat. No. 3,471,371 (1969); and H. A. Fromson U.S. Pat. No. 3,929,594 (1975); U.S. Pat. No. 4,021,592 (1977); and U.S. Pat. No. Re. 29, 754 (1978). In liquid contact, the DC current is applied to an anode which is dipped in an electrolyte, the so-called liquid contact cell, before or after the anodizing tank. The anode is kept 5 inches away from the aluminum web in a liquid contact cell in which the electrolyte is usually the same as used in the anodizing tank. The liquid contact solution can be a separated tank or a compartment in the anodizing tank. However, the anode used in the liquid contact section oxidizes quickly. The material of anode used in liquid contact is usually chemical lead and lead alloys. The oxides formed on the lead anode will deteriorate and contaminate the electrolyte. No anodes have been found satisfactory enough to be an anode in the liquid contact cell.
While the liquid contact treatment of aluminum webs in a sulfuric acid electrolyte according to known procedures does yield an aluminum web having advantageous characteristics for its use as a printing plate substrate, these procedures do entail certain drawbacks. One such substantial drawback is the failing of the anodic electrode employed in the liquid contact treatment due to the effect of the highly corrosive sulfuric acid electrolyte employed. Various suggestions have been made to overcome this disadvantage, including the employment of special electrodes which have been alleged to be resistant to anodic attack in the presence of a sulfuric acid electrolyte. For example, suggestions have been made to employ a titanium electrode or an electrode of the platinum group of metals, as in British Pat. No. 1,206,863.
While some of the suggested anodic electrode compositions suggested by the prior art have been successively employed in the electrochemical treament of chloride-containing electrolytes, none have been found to provide satisfactory results in the case of a sulfuric acid electrolyte. Many of the anodes suggested had unsatisfactory life-spans in the sulfuric acid electrolyte and failed in a very short time.
The term "lithographic-grade aluminum webs" as used in this specification and claims is intended to encompass aluminum webs, coils and sheets which are useful for, and manufactured expressly for, the production of lithographic printing plates. Such Aluminum Association Alloys as 1100, 3003 and 1050 have been found to be very suitable for this purpose.